Malignant tumors that are intrinsically resistant to conventional therapies represent significant therapeutic challenges. Such malignant tumors include, but are not limited to malignant gliomas and recurrent systemic solid tumors such as lung cancer. Malignant gliomas are the most abundant primary brain tumors, having an annual incidence of 6.4 cases per 100,000 (CBTRUS, 2002-2003). These neurologically devastating tumors are the most common subtype of primary brain tumors and are one of the deadliest human cancers. In the most aggressive cancer manifestation, glioblastoma multiforme (GBM), median survival duration for patients is 14 months, despite maximum treatment efforts. A prototypic disease, malignant glioma is inherently resistant to current treatment regimens. In fact, in approximately ⅓ of patients with GBM the tumor will continue to grow despite treatment with radiation and chemotherapy. Median survival even with aggressive treatment including surgery, radiation, and chemotherapy is less than 1 year (Schiffer, 1998). Because few good treatment options are available for many of these refractory tumors, the exploration of novel and innovative therapeutic approaches is important.
Gene therapy is a promising treatment for tumors, including gliomas, and the identification of genetic abnormalities contributing to malignancies is providing important information to aid in the design of gene therapies. Genetic abnormalities indicated in the progression of tumors include the inactivation of tumor suppressor genes and the overexpression of numerous growth factors and oncogenes. Tumor treatment may be accomplished by supplying a polynucleotide encoding a therapeutic polypeptide or other therapeutic that targets the mutations and resulting aberrant physiologies of tumors. It is these mutations and aberrant physiologies that distinguish tumor cells from normal cells. A tumor-selective virus is an especially promising tool for gene therapy, and recent advances in the knowledge of how viruses replicate have been used to design tumor-selective oncolytic viruses.
In gliomas, several kinds of conditionally replication competent viruses have been shown to be useful in animal models, for example: reoviruses that can replicate selectively in tumors with an activated ras pathway (Coffey et al., 1998); genetically altered herpes simplex viruses (Martuza et al., 1991; Mineta et al., 1995; Andreanski et al., 1997), including those that can be activated by the different expression of proteins in normal and cancer cells (Chase et al., 1998); and mutant adenoviruses that are unable to express the E1B55 kDa protein and are used to treat p53-mutant tumors (Bischof et al., 1996; Heise et al., 1997; Freytag et al., 1998; Kim et al., 1998). Taken together, these reports confirm the relevance of oncolytic viruses (OVs) as anti-cancer agents. In all three systems, the goal is the intratumoral spread of the virus and the ability to selectively kill cancer cells. Along with directly killing the cancers cells, agents that can also influence the microenvironment surrounding the tumor may enhance the therapeutic effect of the OV.
Replication selective oncolytic viruses have shown great promise as anti-tumor agents for solid tumors. The viruses have been constructed genetically so that they are able to preferentially replicate within tumor cells, while being at least somewhat restricted in their ability to replicate in normal cells. The principal anti-tumor mechanism of oncolytic viruses is through a direct cytopathic effect as they propagate and spread from initially infected tumor cells to surrounding tumor cells, achieving a larger volume of distribution and anticancer effects. Oncolytic herpes simplex viruses (HSVs) were initially designed and constructed for the treatment of brain tumors. Subsequently, they have been found to be effective in a variety of other human solid tumors, including breast, prostate, lung, ovarian, colon and liver cancers. The safety of oncolytic HSVs has also been extensively tested in mice and primates, which are extremely sensitive to HSV.
HSV-1 based oncolytic viruses are particularly promising because of: (1) their ability to infect a wide variety of tumors; (2) their inherent cytolytic nature; (3) their well-characterized large genome (152 Kb) that provides ample opportunity for genetic manipulations wherein many of the non-essential genes can be replaced by therapeutic genes; (4) their ability to remain as episomes that avoid insertional mutagenesis in infected cells; and (5) the availability of anti-herpetic drugs to keep in check possible undesirable replication.
Despite encouraging preclinical studies, results from early clinical trials have suggested that most of the current versions of oncolytic viruses, although acceptably safe, may only have limited anti-tumor activity on their own. While not wishing to be bound by any one particular theory, one of the main reasons for the sub-optimal oncolytic efficacy is probably because viral gene deletions that confer tumor selectivity also result in reduced potency of the virus in tumors. For example, the complete elimination of endogenous γ34.5 function from HSV, one of the common approaches for the construction of oncolytic HSV, significantly reduces viral replication potential and therefore may compromise the ability of the virus to spread within the targeted tumors (Kramm et al., 1997).
Considering the limited effective treatment options available for certain types of cancer, including certain types of brain cancer, there remains a need in the art for improved oncolytic viruses.